It is possible that television broadcast is roughly classified, according to its contents, into “main program broadcast” or “commercial messages broadcast”. The main program broadcast is, if it is a movie program, audio-video data of the movie's contents itself. On the other hand, the commercial messages broadcast is audio-video data that is provided by clients, such as a sponsor and an advertising agency making a contract with a broadcaster, to be broadcast during breaks of the main program broadcast. A broadcaster may sometimes perform its own advertisement. Commercial Messages are generally abbreviated as “CM”, which will be thus used in this document. In many cases, content of the main program broadcast less relates with that of the CM broadcast, thus there has been a demand that only the main program broadcast in a video-recorded television broadcast be continuously watched, while skipping CM broadcast. On the other hand, many of recent CM broadcasts fully use the latest imaging technique, or have a high artistic quality or a high originality, which leads to another demand that only CMs be watched. In order to serve the demands, a technique is necessary that automatically distinguishes the CM broadcasts from the main program broadcasts with high accuracy.
In order to distinguish the CM broadcast in a television broadcast, there has been a technique that detects, for example, an audio format in television broadcast to thereby automatically distinguish the CM broadcast from the main program broadcast. The technique determines a stereophonic-sound detected period as a CM broadcast period, using a feature that a monaural sound or a binaural sound (Japanese and English in foreign movies or the like) is typically used for the main program broadcast as its audio format, while stereophonic sound is typically used for the CM broadcast.
There has been another technique in which a CM-broadcast period is found out by detecting an frame image with the whole image being black, named as “a black frame”, that is inserted at a boundary between the main program broadcast and the CM broadcast (for example, refer to Patent document 1). There has also been another technique in which a silence period occurring at a boundary between the main program broadcast and the CM broadcast is detected so that a CM-broadcast period is found out on the basis of intervals of silence periods occurring (for example, refer to Patent document 2).    [Patent document 1] U.S. Pat. No. 5,696,866, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. H08-507633 (page 12-15, FIG. 1)    [Patent document 2] Japanese Unexamined Patent Application Publication No. 2003-47031 (Page 5-8, FIG. 1)
According to the CM-broadcast periods thus recognized by these techniques, the recorded television broadcast can be played back with the CM-broadcast periods being skipped, by referring time information, for example, of the CM-broadcast periods that has been stored into a recording medium.
These techniques distinguishing a CM broadcast from a television broadcast make use of various kinds of transition points in images and/or sounds in television broadcast to automatically recognize a CM broadcast. However, transition points of the previously-mentioned “black frames” or the silence periods may occur even during the main program broadcast other than CM broadcasts, and, depending on stage effects in the program, the transition points may frequently occur.